Reset circuits are used to monitor power supplies in microprocessors, digital equipment, and various other electronic equipment and systems. A reset circuit is used to assert a reset signal whenever the supply voltage falls below a determined threshold voltage and to de-assert said reset signal when the supply voltage rises above the threshold. This reset signal may be input to the microprocessor, for example, to start the microprocessor in a known state during power up to prevent code execution errors, during power down to initiate a clean shutdown sequence, and during brownout to achieve control over marginal voltage conditions.
One common deficiency of reset circuits is that proper assertion of reset signals during power up and power down conditions does not reliably occur. As voltages become low during power down, reset may be asserted intermittently due to reset circuitry voltages which are below minimum operational limits. Similarly, during power up reset stabilization may not occur until reset circuit voltages exceed minimum operational levels.
During power down, an accurate reset assertion may be latched when power down is first detected to ensure an orderly shutdown. During power up, however, circuitry may not work in a predictable or reliable manner when the supply voltage is low, and reset assertion is problematic.